1. Field of the Invention
This invention relates to the field of atomic layer deposition (“ALD”), and more particularly to apparatus and methods for performing ALD with high throughput and low cost.
2. Description of Prior Art
Thin film deposition is commonly practiced in the fabrication of semiconductor devices and many other useful devices. An emerging deposition technique, atomic layer deposition (ALD), offers superior thickness control and conformality for advanced thin film deposition. ALD is practiced by dividing conventional thin-film deposition processes into single atomic-layer deposition steps, named cycles, that are self-terminating and deposit precisely one atomic layer when conducted up to or beyond self-termination exposure times. The deposition per cycle during an ALD process, the atomic layer, typically equals about 0.1 molecular monolayer to 0.5 molecular monolayer. The deposition of atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and eliminates the “extra” atoms originally included in the molecular precursor.
In ALD applications, typically two molecular precursors are introduced into the ALD reactor in separate stages. Adequate ALD performance requires that different molecular precursors are not allowed to intermix within the deposition chamber, at the same time. Accordingly, the reaction stages are typically followed by inert-gas purge stages that eliminate the molecular precursors from the chamber prior to the separate introduction of the other precursor.
During the ALD process, films can be layered down in equal metered sequences that are all identical in chemical kinetics, deposition per cycle, composition, and thickness. This mechanism makes ALD insensitive to transport nonuniformity resulting in exceptional thickness control, uniformity and conformality.
If ALD is to become commercially practical an apparatus capable of changing the flux of molecular precursors from one to the other abruptly and fast needs to be available. Furthermore, the apparatus must be able to carry this sequencing efficiently and reliably for many cycles to facilitate cost-effective coating of many substrates. A useful and economically feasible cycle time must accommodate a thickness in a range of about from 3 nm to 30 nm for most semiconductor applications, and even thicker films for other applications. Cost effectiveness dictates that substrates be processed within 2 minutes to 3 minutes, which means that ALD cycle times must be in a range of about from 5 seconds to 0.5 seconds and even less. Multiple technical challenges have so far prevented cost-effective implementation of ALD systems and methods for manufacturing of semiconductor devices and other devices.
Given the need for short cycle times, chemical delivery systems suitable for use in ALD must be able to alternate incoming molecular precursor flows and purges with sub-second response times. The need to achieve short cycle times requires the rapid removal of these molecular precursors from the ALD reactor. Rapid removal in turn dictates that gas residence time in the ALD reactor be minimized. Gas residence times, τ, are proportional to the volume of the reactor, V, the pressure, P, in the ALD reactor, and the inverse of the flow, Q, τ=VP/Q. Accordingly, lowering pressure (P) in the ALD reactor facilitates low gas residence times and increases the speed of removal (purge) of chemical precursor from the ALD reactor. In contrast, minimizing the ALD reaction time requires maximizing the flux of chemical precursors onto the substrate through the use of a high pressure within the ALD reactor. In addition, both gas residence time and chemical usage efficiency are inversely proportional to the flow. Thus, while lowering flow will increase efficiency, it will also increase gas residence time.
Existing ALD apparatuses have struggled with the trade-off between the need to shorten reaction times and improve chemical utilization efficiency, and on the other hand, the need to minimize purge-gas residence and chemical removal times. Thus, a need exists for an ALD apparatus that can achieve short reaction times and good chemical utilization efficiency, and that can minimize purge-gas residence and chemical removal times.
Existing ALD apparatuses have also struggled with performance deterioration caused by extensive growth of inferior films on the walls of the ALD chambers. This performance deterioration facilitated short equipment uptime and high cost of maintenance. Thus, a need exists for an ALD apparatus that can minimize the growth of deposits and minimize their impact on performance therefore facilitating substantially longer uptime and reduce the cost of maintenance.
Existing ALD apparatuses have struggled with performance deterioration related to slit-valve induced asymmetry with its unavoidable dead-leg cavity. The art of single wafer deposition presents a variety of effective remedies for this problem. For example, U.S. Pat. No. 5,558,717 teaches the advantageous implementation of an annular flow orifice and an annular pumping channel. This annular design requires a relatively wide process-chamber design. In another example, U.S. Pat. No. 6,174,377 describes an ALD chamber designed for wafer loading at a low chuck position, while wafer processing is carried out at a high chuck position, leaving the wafer transport channel, and the flow disturbances associated with it, substantially below the wafer level. Both of these prior art solutions and other prior art solutions are not ideally suited to resolve the slot valve cavity problem in ALD systems.
A better solution implements a ring-shaped slit-valve that creates a substantially symmetric chamber environment. Such embodiment is described in U.S. Pat. No. 6,347,919. However, the ring slit-valve described in U.S. Pat. No. 6,347,919 presents significant performance deterioration that is associated with the presence of unprotected elastomeric seals and the respective crevices between the slide of the ring slit-valve and the chamber wall that is notorious for entrapment of chemicals and the growth of deposits and particulates on the seal and within the crevices. While deterioration of chamber performance related to growth of deposits on slit-valve seals is a universal problem with all existing designs of slit valves, ring-shaped slit-valves as taught in U.S. Pat. No. 6,347,919 substantially aggravate that problem due to substantially longer seals and crevices. Unfortunately, this performance limitation makes the ring-shaped slit-valve that was taught in U.S. Pat. No. 6,347,919 practically unusable for ALD applications.
A substantial improvement that makes ring-shaped and other perimeter slit-valves suitable and advantageous for ALD applications is described in U.S. patent application Ser. No. 10/347575, now U.S. Pat. No. 6,911,092 issued Jun. 28, 2005, which is commonly assigned to Sundew Technologies, Inc., by the inventor of this invention that provides seal and crevice protection during the ALD chemical dose steps, therefore making perimeter slit-valves suitable for Synchronously Modulated Flow-Draw ALD apparatus and method.
Chemical delivery into ALD chambers has been generally been limited to chemicals with substantial vapor pressure. However, many advantageous ALD films rely on molecular precursors that are substantially non-volatile. Accordingly existing ALD systems have struggled with the challenge of consistent chemical delivery of low-volatility molecular precursors as abruptly shaped doses for promoting high productivity ALD processes.
In previous patent applications by the inventor of this invention, U.S. patent application Ser. No. 10/347575, now U.S. Pat. No. 6,911,092 issued Jun. 28, 2005, which is commonly assigned to Sundew Technologies, Inc., and PCT Application No. US03/01548, now PCT Publication No. WO 03/062490 published Jul. 31, 2003, which is commonly assigned to Sundew Technologies, Inc., embodiments that helped solve some of the problems described above were disclosed. Systems, apparatuses, and methods in accordance with that invention provide Synchronous Modulation of Flow and Draw (“SMFD”) in chemical processes, and in particular, in atomic layer deposition processes and systems. These patent applications are included here as references.
Atomic layer deposition (“ALD”) is preferably practiced with the highest possible flow rate through the deposition chamber during purge, and with the lowest possible flow rate during dosage of chemicals. Accordingly, an efficient ALD system in accordance with U.S. patent application Ser. No. 10/347575, now U.S. Pat. No. 6,911,092 issued Jun. 28, 2005, which is commonly assigned to Sundew Technologies, Inc., and PCT Application No. US03/01548, now PCT Publication No. WO 03/062490 published Jul. 31, 2003, which is commonly assigned to Sundew Technologies, Inc., is able to generate and accommodate
significant modulation of flow rates. Under steady-state conditions, the flow of process gas (either inert purge gas or chemical reactant gas) into a chamber, referred to herein as “flow”, substantially matches the flow of gas out of a chamber, referred to herein as “draw”.
An important aspect of an embodiment in accordance with the invention described in U.S. patent application Ser. No. 10/347575, now U.S. Pat. No. 6,911,092 issued Jun. 28, 2005, which is commonly assigned to Sundew Technologies, Inc., and PCT Application No. US03/0 1548, now PCT Publication No. WO 03/062490 published Jul. 31, 2003, which is commonly assigned to Sundew Technologies, Inc., is that it resolves the trade-off in conventional ALD systems between the contradictory requirements of a high flow rate during a purge of the deposition chamber and of a low flow rate during chemical dosage. SMFD in accordance with that invention provides the ability to purge a process chamber at a low-pressure and a high purge-gas flow rate, and sequentially to conduct chemical dosage in the process chamber at a high-pressure and a low flow rate of chemical reactant gas, and to modulate pressures and gas flow rates with fast response times.
While the SMFD-ALD device disclosed in the prior applications of this inventor is a significant improvement in the deposition art, in some respects the design still reflects the technology available in conventional deposition processes it would be highly useful to have an improved process chamber that takes better advantage of the SMFD process, and more fully develops chemical utilization efficiencies. It also would be useful to provide SMFD-ALD chemical source designs better adapted to the SMFD process, especially for the efficient and consistent delivery of vapor from low-volatility liquid and solid chemicals.